1. Field of the Invention
The present invention relates to an optical element for special-effect exposure which is attached to a photographic lens, a video camera lens or the like or which is used as a component of a lens and more particularly a soft-focus lens best adapted to obtain an image with soft gradation.
2. Related Background Art
In order to obtain portraits, landscape pictures and the like all of which have a soft gradation, soft-focus lenses designed and constructed for obtaining images with soft gradation have been used. These soft-focus photographic lenses have a high degree of soft-focus effect and a high degree of beautiful vignette, but they are highly expensive, and the effect of soft focus varies over a wide range when a photographic lens is set to a low aperture so that in order to attain a high degree of soft-focus effect, special photographic techniques are required. Therefore, instead of the above-described soft-focus photographic lenses, various types of soft-focus filters have been proposed and demonstrated so that when they are attached to the conventional sharp-focus lenses, photographic pictures and images with soft gradation can be obtained.
Various types of soft-focus filters are available. For example, in one soft-focus filter, a plurality of fine coaxial grooves are formed on the light-transmitting surface of the filter. In another type, a large number of great and small frosted projections and recesses are defined on a light-transmitting surface. In a further filter, a plurality of lens-like projections are extended in a predetermined regular pattern from a light-transmitting surface. In yet another type, a plurality of projected phase portions are defined so as to attain phase differences from a light-transmitting surface. In yet a further type, a surface of glass is formed with spots each having a different refractive index by an ion exchange process. The following conditions must be satisfied even when such soft-focus filters are attached to the photographic lenses in order to obtain a satisfactory degree of soft-focus effect as in the case of the conventional soft-focus photographic lenses:
(1) In response to the resolving power of a photographic lens upon which a soft-focus filter is mounted, sharply focused images must remain faithfully at each image-focusing point.
(2) A degree of vignette of a focused picture must be high and it must not produce a ring-shaped vignette around a bright point image and straight-line-like flashes of light around a bright configuration as in the case of the so-called two-line vignette.
(3) The range and the degree of blurs due to the vignette of an image are within a satisfactory range, respectively.
There has not yet been proposed, however, a soft-focus filter which can satisfy all of the conditions described above. For instance, in the case of a filter having a plurality of coaxial grooves at the light-transmitting surface, a ring-shaped vignette occurs around a bright image of a point light source so that the adverse effects of the so-called two-line vignette are aggravated, and consequently it is becomes impossible to obtain beautiful vignette. In a case of a filter of the type in which a large number of frosted projections and recesses are defined on the light-transmitting surface, the contrast of the whole image is degraded due to the scattered light rays emitted from the surface with projections and recesses, so that the resolving power of an image is degraded and consequently a soft-focus image with natural vignette cannot be obtained. Furthermore, in the case of a conventional filter of the type having a plurality of lens-shaped projections extended from the light-transmitting surface, water-drop-shaped projections each having a relatively large diameter are regularly arrayed over the whole surface of the light-transmitting surfaces so that vignette around the image of a point light source is extremely evident and does not constitute a gentle blur. As a result, ring-shaped vignette tends to occur, so that the so-called degree of vignette is degraded.
Furthermore, in the case of a phase difference type filter, the degree of vignette is dependent upon the wavelengths of light rays, so that the decrease in contrast can only be obtained in response to a specific wavelength of light; and furthermore the tendency of generating the so-called two-line blur is increased, whereby beautiful vignette effects cannot be attained. In addition, in order to obtain a predetermined phase difference, the lens surface must be finished with an extremely high degree of dimensional accuracy, so that the fabrication of such filters is difficult and is very expensive. In the case of a soft-focus filter fabricated by an ion exchange process, the soft-focus effect substantially similar to that obtained by a soft-focus photographic lens can be attained, but there is the problem that only specific materials can be used and the production cost becomes high.